The application generally relates to a motor drive for a permanent magnet motor. The application relates more specifically to a method and system for controlling a high speed permanent magnet synchronous motor (PMSM).
Variable speed drives (VSDs) are used to power a variety of motor types in Heating, Ventilation, Air Conditioning and Refrigeration (HVAC&R) systems. Common types of motors that are used for HVAC&R systems include induction motors, switched reluctance motors, and other AC and DC motors capable of handling the torque and speed ranges required in such HVAC&R systems.
Permanent magnet synchronous motors (PMSMs) are of particular interest for use in HVAC&R systems due to their higher efficiency and higher power density as compared to regular AC induction motors. PMSMs are rotating electric machines that operate with a permanent magnet rotor. A permanent magnet rotor may be configured with surface mounted permanent magnets or with interior permanent magnets having different configurations or arrangements. The stator of a PMSM may be similar to a stator of an induction motor. However, a totally different stator design for a PMSM is possible and stator design optimization may be necessary even though the stator topology might be similar to an induction motor. The use of a permanent magnet to generate a substantial air gap magnetic flux makes it possible to design highly efficient PMSMs.
A PMSM driven by a sinusoidal current is referred to as a PMSM while, a PMSM driven by a rectangular phase-current waveform can be referred to as a brushless DC (BLDC) machine. The rotor structure of the PMSM and BLDC can be the same as a surface-mounted permanent magnet rotor. Both the PMSM and BLDC are driven by stator currents coupled with the given rotor position. The angle between the generated stator flux linkage and the rotor flux linkage, which is generated by a rotor magnet, defines the torque, and thus speed, of the motor. Both the magnitude of the stator flux linkage and the angle between the stator flux linkage and rotor flux linkage are controllable to maximize the torque or minimize the losses. To maximize the performance of PMSM and ensure the system stability, the motor requires a power electronics converter for proper operation.
In order to achieve maximum performance and control when operating a PMSM it is necessary to determine the rotor position. Speed or position sensors, or a combination of both, can be used to determine the rotor position. However, speed or position sensors may not perform properly when exposed to a harsh environment. The addition of sensors also increases the system cost, and may require a complete disassembly of the PMSM in the event of a sensor failure.
For high speed and ultra-high speed applications, special speed and position sensors are required, and availability and cost of special speed and position sensors may be a problem. Various kinds of sensorless schemes have been proposed to remove the speed or position sensor by estimating the position from a measured electrical variable, for example, by obtaining the rotor position information from the flux linkage. Accurate flux estimation is required for the rotor position estimation in the sensorless control of PMSM. The existing methods for flux estimation may be adequate when the ratio of the switching frequency to the fundamental frequency is high and the sampling frequency to fundamental frequency ratio is high. However, when operating at a low sampling frequency to fundamental frequency ratio and low switching frequency to fundamental frequency ratio, which is normally the case for high speed or ultra-high speed PMSM drives, accurately estimating the flux linkage becomes more difficult. Thus the traditional methods are not applicable.
Intended advantages of the disclosed systems and/or methods satisfy one or more of these needs or provide other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.